1. Field of the Invention
This invention relates to a method for manufacturing a glass preform and a glass fiber using a chalcogenide glass or oxychalcogenide glass and to a glass preform and a glass fiber using a chalcogenide glass or oxychalcogenide glass. The glass fiber can contain light emitting substances in the core of the glass fiber and is useful as a fiber for optical amplification. This invention also relates to an optical fiber amplifier.
2. Description of Related Art
In optical telecommunication fields, 1.3-micron band optical amplifiers inexpensive and operational with high efficiency have been desired these days. A 1.3-micron band optical amplification medium currently used is an optical fiber, in the core of which Pr.sup.+3 ions are added as light emitting substances. A chalcogenide glass is expected to make a host glass to which the Pr.sup.+3 ions are added. Using such a chalcogenide glass allows makers to create an amplifier having very high efficiency.
To obtain an amplifier having further high efficiency, however, it is required to make sure that the glass contain light emitting substances uniformly while the substances are at an ionic state. The chalcogenide glass generally has property to render the ionic substances as light emitting substances hardly soluble in the chalcogenide glass. We therefore have paid attentions to metal sulfide chalcogenide glasses, constituted essentially of sulfur as a chalcogen element, serving as host glasses surely rendering ionic substances more soluble. Such metal sulfide chalcogenide glasses, because of the high solubility of ionic substances, allow the ionic substances to be doped in a relatively large amount in the glass. The metal sulfide chalcogenide glasses are expected to replace arsenic-sulfuric glasses that are currently manufactured in a large number as chalcogenide glass fibers but have low solubility of ionic substances.
To use the glass as an optical amplifier medium, the glass is required to be fabricated into a form of a single-mode optical fiber. A known method to form the chalcogenide glass into a fiber is a method based on a pot method as disclosed in, e.g., Japanese Unexamined Patent Publication, Showa 64-3,031.
The method disclosed in Japanese Unexamined Patent Publication, Showa 64-3,031, because of the pot method, is not suitable for fabricating a fiber having a core of 15 to 16 micron meters or less in diameter, which is particular for single-mode fibers. Ordinary methods for fabricating a single-mode fiber is a preform method in which a rod-in-tube, or extrusion molding, etc., forms a preform having a larger ratio of a core diameter to a clad diameter and then a part of the preform is heated and softened to make the preform extended.
The metal sulfuric chalcogenide glasses, however, tends to have a lower stability against crystallization than the arsenic-sulfuric glass and may lose mechanical strength of the glass due to foreign objects or latent scratches. When the metal sulfuric chalcogenide glass is drawn into a fiber, the glass surface may be crystallized around the foreign objects or latent scratches as nucleuses on the glass surface, and if crystallized once, the glass is hardly made into a fiber. It was impossible in a practical sense to fabricate an optical fiber in applying a conventional preform method as it was. That is, because in the preform method, at a time of the drawing, a side face of the preform rod or of the jacketing tube makes the side face of the fiber as it is, the fiber's mechanical strength is greatly reduced if foreign objects cling to or latent scratches due to polishing exist on the surface of the preform or tube. Due to this ground, no example has been known in which a practically useful fiber is successfully made by a drawing method, although the sulfuric chalcogenide glass can be doped with a relatively large amount of light emitting substances.
Generally, polishing or etching surfaces is widely used as methods for removing metamorphic layers residing on the glass surfaces and foreign objects clinging to the glass surfaces. Polishing to remove metamorphic layers and foreign objects causes latent scratches, and such latent scratches would unavoidably remain on the surfaces. If foreign objects cling to the glass surfaces or if latent scratches produced due to polishing reside on the glass surfaces, the mechanical strength of the glass would be greatly reduced.
Etching is implemented for removing foreign objects, latent scratches, etc. from the glass surface to form a non-oxide glass having lesser stability of crystallization, e.g., fluoride glass into a fiber. For example, U.S. Pat. No. 4,631,114 discloses removal of metamorphic layers, foreign objects, and latent scratches on a preform or tube surface by etching with a special etchant over the preform or tube surface.
Necessary property of the etchant is: first, the etchant will not increase micro undulation on the glass surface after removing the metamorphic layers, foreign objects, and latent scratches on the glass surface; second, the etchant will not promote latent scratches; and third, the etchant will not create any new metaphoric layer. When etching is made using an etchant dissatisfying those first to third conditions, the strength of the fiber may be deteriorated more than prior to the etching.
A chalcogenide glass, in particular, a sulfide glass, if inadvertently dipped in an etchant including an acid, generates poisonous hydrogen sulfide, etc., in accompanied with solution of the glass, and creates risks. Therefore, an etchant should be designed in the light of compositions of the glass as an etching target.
No etchant has been known so far which satisfies the first to third conditions above and which is designed in consideration of safeness as well. Accordingly, nobody knows a method for forming sulfide chalcogenide glass into a fiber upon removal of foreign objects clinging to or latent scratches created during polishing on the surfaces of the preform rods or jacketing tubes.
As separated from above problems, chalcogenide elements such as sulfur or the like volatilize from the side faces of the preform rods or jacketing tubes directly exposed in a gas phase atmosphere. Particularly, for the sulfuric chalcogenides, such volatilization of sulfur is remarkable. When the chalcogenide elements such as sulfur and the like volatilize, the surface composition of the preform rods or the jacketing tubes may shift, or become different, from the inside. The metal sulfuric chalcogenide glasses, as described above, have a narrower glass range and inferior stability against crystallization in comparison with the arsenic-sulfuric glasses currently used widely as chalcogenide glasses. If the chalcogenide elements such sulfur and the like volatilize overly from the surfaces of the preform rods and jacketing tubes, compositional shifts at the surface induced by the volatilization cause the surfaces to be very easily crystallized. It is therefore desired, when necessary, to suppress such crystallization caused by the volatilization of the chalcogenide elements, other than foreign objects, etc., clinging to the surfaces.